1. Field of the Invention
The present invention relates to adaptive beam forming techniques for controlling the beam pattern of a transmission beam. In particular, but not exclusively, the present invention relates to adaptive beam forming techniques for use in the downlink of a mobile cellular communications system, and to hybrid transmit diversity and adaptive beam forming techniques.
2. Description of Related Art
In a cellular mobile communications system, the area in which the system is to operate is divided into a plurality of cells, each of which is served by a base station. Each base station is allocated a certain number of radio frequencies which are used to transmit signals to and receive signals from mobile units in the cell. Those radio frequencies are reused in other non-adjacent cells, thereby increasing the spectral efficiency of the system.
Within each cell, it is necessary for the base station to transmit to each wanted user (i.e. each active mobile station) in a multi-user and multi-path environment. In order to achieve satisfactory signal detection at low bit error rates, two conditions must be satisfied. Firstly, the power level of the signal received by the mobile unit from the base station must be greater than a certain threshold value. Secondly, the multi-user interference (MUI), sometimes referred to also as multiple access interference (MAI), must be reduced to an acceptable level.
In order to reduce MUI while keeping signal power levels above the threshold values, it is known to divide cells into a number of sectors, each of which is served by a directional antenna. For example a cell may be divided into three sectors by using three directional antennas spaced apart at angular intervals of 120°. In this way, when transmitting to mobile unit, a base station need only provide coverage for the sector in which the mobile unit is located, rather than the whole cell.
In the system described above there may still be many mobile units in a sector, which may lead to unacceptable levels of MUI. If an attempt were made to reduce the number of mobile units in any one sector by reducing the sector size, this would lead to an increase in the number of handovers from one sector to another which would need to be performed, which would reduce the efficiency of the system.
In an article by Y. J. Guo et al entitled “Advanced base station technologies for UTRAN”, Electronics & Communications Engineering Journal, June 2000, the entire content of which is incorporated herein by reference, a technique for adaptive beam forming is disclosed, in which the beam pattern of a downlink transmission beam is adapted by determining the direction of reception of an uplink transmission beam. Since the downlink and uplink transmission beams may not have the same channel characteristics, for example because different frequencies are used for the uplink and the downlink, this technique may not always provide reliable results.
In order to overcome the problem of MUI in the uplink of a mobile communications system, it is known to employ adaptive antennas at base stations which spatially filter received signals. For example, United Kingdom patent application number 9826271.0 in the name of Fujitsu Limited, the entire content of which is incorporated herein by reference, discloses an adaptive antenna in which a beam former is used to spatially filter a transmission signal sent to a base station by a mobile unit. An adaption algorithm is employed which adjusts the beam former response in dependence on an error signal such that the beam former tracks the transmission signal of a particular mobile unit as the mobile unit moves within the cell.
If an adaptive antenna analogous to that described above were to be used in the downlink of a cellular mobile communications system, complex feedback information would have to be sent from the mobile unit to the base station, which would lead to an undesirable increase in the amount of information which would have to be transmitted in the uplink. Furthermore, the applicable standards for the system may not provided for the uplink transmission of such feedback information.
It is therefore desirable to provide an adaptive antenna suitable for use in the downlink of a cellular mobile communications system. In particular it is desirable to provide an adaptive antenna which avoids the need for complex feedback information to be transmitted from a mobile unit to a base station.